Being Overweight :(

Hi All! I'm Rachael, I am currently 17 years old. I am 140 kg... i know its too much, i keep on gaining and was always bullied at school..... My parents decided to bring me to the doctors and they said that i have hypothyroidism. I am very scared... they have thought me some ways like diet n exercise, first 2 weeks i lost 1kg the next 2 weeks i gain 1.5 kg. i just couldn't help..., i love dancing, i dance recently rather than that i dont exercise because i work in a office n sit for 9 hours+. My mum have gave me till the end of the year to lose weight if not she is gonna make me go for surgery. I am very very scared for surgeries and it has a lot of complications.Please help and advise me to lose weight easily...and any testimonials of how u lose weight? Thank u. :)